1. Field of the Invention
The present invention generally relates to an optical element that can be used as a resonant filter capable of resonating and reflecting light having a specific wavelength and an optical filter capable of transmitting light having a specific wavelength, to a refractive index sensor and a refractive index sensor array that detect a reaction and a change in a refractive index of a measurement sample by using the optical element, and to a biosensor that checks an antibody reactivity and the like by using the refractive index sensor array. Moreover, the optical element of the present invention can be applied to a polarization sensor that detects a polarization direction of light.
2. Description of the Related Art
In recent years, with the development of biotechnologies, a demand for a biosensor that has high sensitivity and can be manufactured at low cost has been increasing. In this circumstance, a sensor and a device that detect a change in a refractive index have been expected as a sensor for detecting an antigen-antibody complex reaction, a sensor for detecting a change of a component in a specific object or liquid, and a sensor for monitoring a component in blood and urine.
As such a sensor, a sensor using a surface plasmon resonance has been known. Currently, detections are often conducted by labeling a deoxyribo nucleic acid (DNA) or protein with a fluorescence substance and the like and reading the coupled fluorescence substance by a fluorescence microscope or a fluorescence scanner. Since there are advantages in that the preceding step of labeling a fluorescence substance can be omitted and no adverse effect of the preceding step is caused on the DNA, protein, and other substances, a measurement method using the surface plasmon resonance has been actively researched. The surface plasmon resonance is a phenomenon in which surface plasmon waves induced on a metal surface when light is incident to the metal layer is excited by resonating with evanescent waves generated by the incident light. A representative optical structure of a sensor using such a surface plasmon resonance employs total reflection of a prism. To be specific, when light is incident on a prism on which a metal film is deposited by evaporation, evanescent waves generated on a surface of the prism and surface plasmon waves excited on the metal surface resonate with each other. This resonance is called a surface plasmon resonance. An incident angle that causes the surface plasmon resonance changes depending on a refractive index of a sample provided on the metal surface. By utilizing this phenomenon, a change in an angle of the incident light at which the reflected light is reduced has been detected as a change in a refractive index of the surface (see Patent Documents 1 and 2).
In Patent Document 3, the surface plasmon resonance is used as well and has a configuration in which an incident light source, a prism, and a photodetector are fixed. Accordingly, there is no need to adjust a positional relationship among the devices after the sensor is completed. A fixed CCD (Charge Coupled Device) imaging element serves as the photodetector to detect a change in an amount of reflected light of each reflection angle in order to detect a change in the refraction index.
Further, Patent Document 4 discloses a sensing method utilizing a resonant reflection (resonance with reflection) generated when incident light resonates with a structure having a protrusion and a recession, whereby the size of the structure is equal to or less than a wavelength of the incident light. Various modes can be considered to cause this resonant reflection. A simple principle to cause the resonant reflection is shown in FIG. 3 by using a general configuration. A basic structure includes a base material layer 11, a waveguide layer 12 formed of a material with a high refractive index, and a grating layer 13 forming a protrusion-recession periodic structure in which protrusions and recessions are periodically formed. In an area of the grating layer 13, a material part with a high refraction index and a material part with a low refraction index are alternately formed with a predetermined period.
FIG. 4 is a graph showing a transmission factor of light with respect to a wavelength of incident light in the case where a period and a refractive index of the above-described structure are optimized. This shows that incident light resonates and is reflected at only a certain specific wavelength λ1 with the structure having a specific period and refractive index in a size equal to or less than a wavelength of the incident light. These components are all formed of a material that is transparent with respect to the incident light. Therefore, most of the incident light is transmitted except that only the above-described condition of the structure having the specific period causes resonant reflection of the incident light.
By using the phenomenon of resonant reflection, a change in a refractive index of the measurement sample can be detected by irradiating an element having a measurement sample with white light and scanning a wavelength of light which resonates and is reflected by a spectroscope. Since labeling with a fluorescence substance and the like is not required in this method either, such problems in that an adverse effect is caused on the substance, the preceding process takes time, and the like can be eliminated. Further, since a high contrast with a high light intensity can be obtained by the resonant reflection, a measurement precision can be enhanced.    [Patent Document 1] Japanese Patent Application Publication No. 5-18890    [Patent Document 2] Japanese Patent Application Publication No. 6-58873    [Patent Document 3] Japanese Patent Application Publication No. 7-225185    [Patent Document 4] Japanese Patent Application Publication No. 2007-506107
However, there have been the following problems in the configurations of the above-described conventional techniques.
By the methods disclosed in Patent Documents 1 and 2, a measurement is performed by utilizing a characteristic of light in that an intensity of reflected light in the total internal reflection changes depending on an incident angle. Therefore, a lens, a prism, and a detector have to be moved at a high positional precision. In order to maintain the positional relationship among these devices, they are required to be fixed by using a member with high rigidity. As a result, an apparatus is enlarged and becomes more expensive.
By the method disclosed in Patent Document 3, the light source, detector, and the like are fixed and a driving part is not provided. Therefore, there is no need of positional adjustment after the setting. Thus, operations can be stabilized. However, since the amount of light of the reflected light at each angle is detected, the CCD used for detection is required to be positioned at a certain distance from the measurement sample. Therefore, this method is not suitable for downsizing and making an apparatus thinner.
By the method disclosed in Patent Document 4, resonant reflection is used for detection, therefore, signals with high contrast can be obtained. However, since a spectroscope is used for this detecting method, it is difficult to downsize the apparatus. Moreover, since only one measurement can be performed with respect to one light source, there are problems in that a process cannot be performed at a higher speed and processes cannot be performed together.
The present invention is made in view of the above circumstances and it is an object of at least one embodiment of the present invention to provide an optical element which causes resonant reflection (resonance with reflection) of light having a specific wavelength by utilizing resonant reflection caused by a periodic structure formed of fine protrusions and recessions in a size equal to or less than a wavelength of incident light. Further, it is an object of at least one embodiment of the present invention to provide a refractive index sensor and a refractive index sensor array which has no moving part; can perform a stable measurement at a high precision; can make an element smaller and thinner; and can make a process speed higher by employing the optical element, and to provide a biosensor that checks an antibody reactivity and the like by using the refractive index sensor array.
To achieve the above objects, following measures are employed in the present invention.